This invention relates to a range limiting system which permits detection of desired backscatter-modulated signals but prevents detection of undesired backscatter-modulated signals and also effectively discriminates against system and radiated noise. More particularly, the system of the invention uses a deliberately injected signal which is combined both with desired and undesired signals to generate a combined signal from which desired backscatter-modulated signals can readily be detected. Systems of this type are described in U.S. Pat. No. 4,739,328 assigned to the same assignee as the subject invention.
The signals used in the invention to be detected come from small backscatterers, called "tags". These tags are located on moving or movable objects, such as vehicles, trains, or shipping containers. In the case of vehicles, for example, it is desirable to obtain specific information relating to the car or truck. The information is contained in the tag mounted on the vehicle, and is transmitted by backscatter modulation from the moving vehicle as it passes through a toll lane. However, other tagged vehicles also are passing through adjacent lanes or moving behind the desired vehicle in the same lane at the same time. If the backscattered signal-collecting antenna is aimed properly, there will be a small difference in signal strength between a desired and an undesired backscattered signal. However, in the presence of other noise, as well as system noise in the detection system itself, these small amplitude differences have been found to be hard to distinguish.
One technique used to improve the discrimination ability of the system is described in U.S. Patent Application Ser. No. 254,254 filed Oct. 6, 1988 and assigned to the same assignee as the subject invention. However, the technique of this invention is an alternative, and may indeed be employed together with the technique of the above-identified patent application to produce the most effective signal discrimination, particularly in areas of heavy signal traffic.